In the meantime, I’ve been playing with KiCad and making my own shield. I got some prototype boards made, and assembled one to test (right). It works perfectly, but the board layout needs a few tweaks.
The beta boards (left) aren’t as gorgeous as the matte black Reactive Substance boards, but I’m partial to the traditional green PCB. The final version will probably be blue. I could not resist some bling, though, so the boards are ENIG (which means they’re nickel-plated and then gold-plated on top). It makes soldering very easy.